1. Technical Field
The invention relates to a method of manufacturing a press felt, in which method a press felt is formed which comprises a body composed of several threads in the transverse direction and in the longitudinal direction and in order to provide a smooth surface with good water retention capacity, a crill layer is needled at least on the surface of the body facing the web to be dried.
The invention further relates to a press felt comprising a body formed of several threads in the longitudinal direction and in the transverse direction, and a crill layer is arranged at least on the surface of the body facing the web to be dried.
2. Description of the Related Art
A press felt is used in presses of paper machines either on one side or on both sides of the web to be dried, depending on the method to be each time applied. Water in the web penetrates the press felt at a compression stage and after the compression the press felt should carry the water without letting it transfer back to the web. In other words, during the compression the paper web is transported on the felt into a slot between two rolls, i.e. into a press roll nip. Typically there are three or four such nips in a row and in each of them water is squeezed out of the web to the felt. In other words, the felt has to be such that the water can easily pass onto the felt compressed in the press roll nip. Prior art press felts comprise a body, i.e. a body texture, providing the press felt with space for water. In addition thereto, crill has been needled to the press felt, the crill providing the desired smoothness of the felt surface and water retention capacity of the felt. In other words, an essential function of the crill is, in addition to marking, to prevent water from transferring back from the felt to the web. The body texture is typically woven. The weaving of the body texture is somewhat slow and costly, for which reason non-woven textures have been developed. U.S. Pat. No. 4,781,967 discloses a multi-layer press felt with a texture comprising non-woven thread arrangements joined crosswise with a smeltable fiber or with glue, the thread arrangements being composed of interlocked modules formed of parallel threads and crill. However, the texture of such a felt is somewhat dense because of the glue or the layer to be melted, used for attaching the threads and crill.
The press felt should be dense enough at the very outset so that the paper machine could be started up rapidly. The most common way to make the press felt dense is to needle fine crill on one or several layers of the felt. The density of the press felt can also be influenced by arranging in the texture a separate compact layer or by making use of a dense texture produced by the bonding texture and/or the shape of the threads. However, such solutions do not function satisfactorily during operation after the start-up, and the dense press felt becomes clogged fast and its operating life remains too short. A short operating life adds to the number of maintenance shutdowns of the paper machine and therefore disturbs the actual production. It is therefore a problem of too dense a press felt that its texture becomes clogged fast, whereby its dewatering capacity and, at the same time, the efficiency of the entire press section become dramatically low. A more loose-textured press felt would, on the other hand, lead to reduced clogging and a sufficiently long operating life of the felt, but, unfortunately, at the same time to a slow paper machine start-up. Such a loose-textured press felt has to be run free of load in the paper machine before the actual production run as long as it has settled and become compressed to provide the desired dewatering properties. If the felt contains too much space for the air carried along and if the liquid transferred onto the felt has space to move, by the impact of compression, to the interior of the felt and not to the grooves or openings of the press roll, dewatering in the nip does not take place in the desired manner. It has also been observed that prior art press felts tend to compress and thereby become excessively dense along with use whereupon they become clogged and their dewatering capacity is reduced.
An object of the present invention is thus to provide a method of manufacturing a new type of press felt, and a press felt with good properties at the very outset of the start up of the paper machine and yet with a sufficiently long operating life attainable.
The method of the invention is characterized in that the press felt is formed in a manner such that during the manufacture of the press felt the texture of the press felt is provided with a portion of a water soluble material which initially makes the texture of the yet uncompressed press felt dense at least for the duration of the start-up of a press section so as to provide the desired dewatering properties, and that at least part of said water soluble material washes away and/or is washed off after the start-up as the initial density provided by the water soluble material is no longer needed, because the texture of the press felt has become dense by the impact of compression.
The press felt of the invention is further characterized in that the texture of the press felt comprises a portion of a water soluble material arranged to provide the desired permeability for the yet uncompressed press felt, and that the press felt is arranged to be installed in a paper machine in a manner such that when being installed, the press felt comprises a portion of said water soluble material arranged to wash away and/or be washed off from the texture of the press felt at least partly later on during the operation of the paper machine as the initial density provided by the water soluble material is no longer needed, because the texture of the press felt has become dense by the impact of compression.
An essential idea of the invention is that the texture of the press felt comprises a water soluble material which makes the texture of the press felt dense for the start-up so that the paper machine can be rapidly made ready for production run after the change of the press felt. It is a further idea that a sufficiently long press felt life can be obtained from the point of view of effective production as the material that makes the press felt dense dissolves gradually and prevents the risk of clogging after the paper machine is restarted after the change of the felt. By the time part of the water soluble material is dissolved from the texture of the felt, the press felt has taken shape, become compressed and changed its properties to reach the desired dewatering properties. The water soluble material is preferably polymer, but other water soluble materials suitable for the purpose can also be used. An essential idea of a preferred embodiment of the invention is that the body is not woven, but it is a texture which is interlocked with thread arrangements bound with the water soluble polymer and to which texture crill is needled. The needling of the crill binds the layers definitively, so that they will hold together even after the water soluble material keeping together the threads of the body before the needling of the crill is substantially entirely dissolved from the texture of the press felt.
An advantage of the invention is that a compromise is no longer needed be made between a rapid start-up capacity and long operating life of the press felt, but the two preferred features can be satisfactorily achieved by the solution of the invention. Such a press felt is of course considerably more effective than the former ones, since it enables the paper machine to be rapidly made ready for production run after the change of the felt and still functions effectively for the normal change interval planned. Consequently, production is no longer disturbed by extra machine shutdowns caused by the change of the press felt. An advantage of the embodiment according to which the body is formed of non-woven thread arrangements is that slow and costly weaving can now be avoided and the body can be manufactured more cost effectively, rapidly and flexibly than before.